Quantification of Weather Effects on Imaging Laser Radar

The Air Force is investigating the use of imaging laser radar systems as autonomous guidance systems on future precision guided munitions. The Air Force\u27s Wright Laboratory is currently testing a 1.06 micron wavelength laser radar (ladar) at Eglin Air Force Base. Since laser radiation can be susceptible to propagation problems through weather, quantitative understanding of these effects is important to the development of this weapon system. Ladar image degradation, in the form of target dropouts and false returns, was evaluated in various weather conditions. Statistical analysis of the images presented expected degrees of degradation according to visibility and rain fall rate. Apparent system noise masked much of the ladar response to weather. It accounted for a dropout percentage of a maximum of 12% of the target, and a false return percentage up to 1 % of the area viewed. Visibility generally less than 2 kilometers, and rain rates greater than 4 millimeters per hour were required to generate dropouts and false returns from the ladar above the system noise level. Heavy rain with rates up to 6.8 inches per hour, and thick fog with visibility down to 200 meters caused the highest percentages of dropouts and false returns. Image degradation showed a generally exponential relation to visibility and those graphs were fitted with best-guess exponential curves. However, dropouts and false returns indicated a strong linear correlation with rain rate. For the heaviest rain, correlation coefficients of 0.91 and 0.90, respectfully, were computed. Best fit linear functions were fitted to the rain rate data